The Octet Meson and Octet Baryon Interaction with strangeness and the $Λ(1405)$

Abstract: The octet meson and baryon interaction with strangeness $S=-1$ is studied fully relativistically with chiral Lagrangian. In this work, a Bethe-Salpeter equation approach with spectator quasipotential approximation is applied to study the reactions $K^-p\to MB$ with $MB=K^-p,\bar{K}^0n, \pi^-\Sigma^+,\pi^0\Sigma^0, \pi^+\Sigma^-$ and $\eta \Lambda$ with all possible partial waves and theoretical results are comparable with experimental data. It is found that the Weinberg-Tomozawa potential derived from the lowest order chiral Lagrangian only provides the contributions from partial waves with spin-parities $J^P=1/2^+$ and $1/2^-$. Two-pole structure of the $\Lambda(1405)$ is confirmed with poles at $1383+99 i$ and $1423+14i$ MeV. The lower and higher poles originate from $\Sigma \pi$ interaction as a resonance and $\bar{K}N$ interaction as a bound state, respectively.